In the generation of dense plasmas, it is customary to use tungsten hairpin type filaments, oxide coatings, or various very broad area cathode configurations to produce the necessary ionizing electrons. Such known plasma cathodes have significant drawbacks or shortcomings. Tungsten hairpin filaments, for example, require large amounts of power, are fragile after heating, and have a limited lifetime. Oxide cathode coatings deteriorate upon exposure to air after use, and are subject to destruction by ion bombardment in dense plasmas. Very broad area cathodes have been proposed in order to achieve the high discharge currents required to produce high density plasmas, but these are wasteful of space and materials.
It has been proposed heretofore to use lanthanum hexaboride as a cathode material. In this connection reference is made to "Boride Cathodes" by J. M. Lafferty, Journal of Applied Physics, Vol. 22 No. 3, March 1951; "Press Sintered Hexaboride Cathodes" by W. Wolski, U. N. Fellowship Studies, No. 69-12, Royal Institute of Technology in Stockholm, Sweden, May 24, 1969; and N. V. Pleshivtsev U.S. Pat. No. 3,798,488. However, despite the known very high current densities which are available from lanthanum hexaboride (LaB.sub.6), the difficulties involved with working with this substance has led most potential users to avoid its use. For example, U.S. Pat. No. 3,798,488 discloses a coating of lanthanum hexaboride on tantalum. Our research indicates that direct contact of the lanthanum hexaboride with tantalum or other refractories including tungsten, causes the embrittlement of the refractory by the migration of boron and the formation of boron-tantalum or other boron-refractory materials, so that the cathode and support structure may have a lifetime of only about 48 hours; and this is confirmed in the patent. Also, the lanthanum is active and will react with other materials such as molybdenum which are frequently used in cathode structures, and cause their failure. In one instance, lanthanum-molybdenum compounds which are formed within the cathode enclosure, even where the molybdenum was spaced from the lanthanum hexaboride, caused failure of the cathode device by the coating of this compound on the cathode and other surfaces within the cathode enclosure, within about 10 hours, using hydrogen as the gas plasma.
Also, at least one authority has indicated that exposure to oxygen is harmful to emissivity. Thus, while the high emissivity properties of lanthanum hexaboride were known, the practical difficulties of using the material, and particularly the adverse reaction with refractories, both in engagement with, and merely exposed near, the cathode, has led those skilled in the art to avoid the material, on the basis that the cathode structure was likely to be very short lived.
Turning to a different phase of the problem, independently of the emitting material chosen, an upper bound on the cathode current is set by the parameters of the surrounding plasma. Under space charge limited conditions, as will be developed in considerable detail in the present specification, the emission current density is proportional to the local plasma density (and to the square root of the electron temperature), Since hundreds of amperes of electron current may be required independent of the local plasma parameters, large area cathodes emitting low current densities have been used in conventional plasma generation designs. These large areas represent a major plasma loss area and have the drawbacks of the conventional cathodes mentioned earlier.
By way of specific example, a typical utilization application might involve a plasma density in the order of 2.times.10.sup.12 electrons per cubic centimeter, and this would be much lower or zero during initial or start-up conditions. The maximum current which can be drawn from the surface of a cathode when exposed to a plasma having a density of 2.times.10.sup.12 electrons, is approximately 5.8 or about 6 amperes per square centimeter, for a typical electron temperature of about three electron volts. For the cathode materials which have generally been used up to the present time, their electron-emitting capability is in the order of a few amperes per square centimeter; accordingly, the relatively low plasma density in the order of 2.times.10.sup.12 electrons per cubic centimeter presented no significant problem.
Lanthanum hexaboride has the capability of emitting very high electron current densities, in excess of 40 amperes per square centimeter, with low evaporation rates. The space charge limitations on compact cathodes using this material becomes critical and an aspect of the invention involves the recognition that special steps must be taken to regulate the plasma density in the vicinity of the lanthanum hexaboride independently of the plasma in the utilization chamber.
Accordingly, one object of the present invention is to utilize the high current emitting capabilities of lanthanum hexaboride to construct compact, high current density cathode structures that are independent of the parameters of the utilized plasma.
Another object of the present invention is to provide a long life geometry for supporting and enclosing a cathode utilizing lanthanum hexaboride as the active emitting element.